Optical shutters use an actuator to drive each of one or more radiation-blocking elements or “shutter blades”, between a first, closed position that blocks the path of light through at least a portion of an aperture and a second, open position that is spaced apart from the first position and that allows light through the aperture. The light radiation that is directed toward the aperture can generally be any form of electromagnetic radiation, such as ultra-violet, visible or infrared radiation, for example. The aperture can be in a frame that is directly or indirectly coupled to the actuator. The frame can additionally support the actuator and typically includes features that retain the shutter blade or blades and that define the travel path of the shutter blade or blades. The actuator can be electromagnetically activated (an “electromagnetic actuator”) so that it responds to an electrical signal to translate the shutter blade or blades in a plane between the open and closed positions. Electromagnetic actuators typically used for this purpose include linear solenoids, rotary solenoids, or brushed or brushless commutated motors, for example.
Actuators for optical shutters can be monostable or bistable. Monostable shutters have a single stable position to which the actuator returns when power is removed. Bistable actuators are able to remain in the last position held at the time power is removed.
Monostable solenoid actuators have a coil of wire that generates a magnetic field when electrical power is applied. The magnetic field applies a force to pull or rotate a soft magnetic core in a given direction. Monostable actuators with soft magnetic cores typically utilize a spring or other mechanical element to return the core to an original position when power is removed. One disadvantage of monostable actuators for shutter control relates to their behavior upon power loss; these actuators require continuous power to remain in the electrically driven state.
Bistable actuators are stable in the state held when power is removed, whether open or closed. Bistable actuators can be created using geared motor drives that lock in a given position when unpowered. In other embodiments, an over-center spring can be used to create a locking force in either of the open or closed positions.
The soft magnetic core of a monostable solenoid can be replaced with a hard magnet that adheres to soft magnetic material in each of its two positions to create a bistable shutter. For example, the rotary drive solenoids (RDS) produced by CVI Melles-Griot are exemplary bistable rotary solenoids, each using a permanent magnet core. Further description of bistable actuators of this type can be found, for example, in Proceedings of SPIE, Vol. 6542, “Advanced electro-mechanical micro-shutters for thermal infrared night vision imaging and applications” by Durfee et al. Bistable actuators are advantaged for their small size and light weight. However, these actuators have their limitations. Because they typically have relatively small coil elements, bistable rotary actuators used for shutter applications can be damaged by the application of continuous power and are typically pulsed intermittently so that energy can be more quickly dissipated. These devices can be constrained in terms of travel arc, allowing the blade to swing over an arc of 20 degrees or less between open and closed positions. This, in turn, tends to limit the size of the aperture.
Stepper motors can alternately be used to drive the shutter blade between the two open and closed positions. U.S. Pat. No. 6,046,519 entitled “Stepping Motor” to Hanazumi et al. provides a description of the structure of conventional “tin can” stepper motor shutters. Hanazumi '519 teaches the steps of providing a permanent magnet with a plurality of poles and sets of pole teeth energized by two coils. The magnet is attached to a shaft. Changing polarity of the electric field in the coils creates an electromagnetic field that works with the magnetic poles to induce rotary motion in a shaft. In an alternate approach, U.S. Pat. No. 5,691,583 entitled “Stepping Motor with Internal Power Connections” to Suzuki et al. describes a different structure for a stepper motor, with the coils centrally located and with the poled magnet in cylindrical form, exterior to the centrally located coils.
Stepper motor-driven shutters are advantaged over solenoid types in that shutter motion can be more closely controlled to reduce shock from impact when the shutter blade is moving between positions. The stepper motor has a rotor that is a permanent magnet having multiple poles or teeth. A set of at least two stators is disposed adjacent to the rotor. The stators have projections that magnetically interact with the magnetic poles or teeth on the rotor. Two coils operate on the stators to generate electromagnetic fields in each of the two stator arms. The fields in the stator arms operate on the poles in the rotor to selectively rotate the rotor from one angular position to the next. The polarity of the two coils can be sequentially changed by reversal of current direction to provide rotation of the stepper motor shaft in either direction.
Stepper motors have an inherent detent torque that provides a small amount of holding force when the stepper motor is de-energized. When the motor is de-energized, the stepper motor shaft settles to a detent angular position where there is maximum attraction between the stator and the poles of the rotor. This detent position has an associated detent torque. The torque needed to move the shaft from this detent position, wherein the torque is generated by applying electrical power to the coils, is significantly higher than the detent torque. In typical stepper motor shutters, a high amount of electrical energy is applied to overcome detent torque and move the blade. Then, once the blade is at a given position, the power to the coils is reduced to provide a holding torque, typically at about half power, that magnetically retains the blade in position after movement.
In some shutter applications, the motion of the blade is relatively infrequent and a minimum of energy is needed for moving the shutter between successive angular positions. In such applications, the stepper motor shutter is generally not the best option, particularly when the blade must be held at a position that is not an un-energized detent position. It is undesirable to apply continuous holding power to the stepper motor when the shutter is not moving but remains in an open or closed position; the need to maintain power when the shutter is stationary wastes energy.
In some applications, there is a need for a system to hold a stepper motor shutter blade in a non-detent position when the stepper motor is not energized. A mechanical holding force can be provided for this purpose; however, this type of solution can require additional components and increased cost, with added concerns for wear and reliability.
Wear and lifetime considerations also relate to operation and parts count. For many types of shutters, a damping apparatus is provided to eliminate or reduce bounce and to help reduce the effects of impact with damper contact at the end of shutter blade travel. However, stops and other damping devices add to parts count and can be wear items.
Thus, it can be seen that there is a need for a stepper motor shutter that has a low parts count and that can be de-energized and still exhibit suitable retention torque.